Beam divergence and various collimators for holographic or stereoscopic displays

ABSTRACT

A holographic display with an illumination device, an enlarging unit and a light modulator. The illumination device includes at least one light source and a light collimation unit, the light collimation unit collimates the light of the at least one light source and generates a light wave field of the light that is emitted by the light source with a specifiable angular spectrum of plane waves, the enlarging unit is disposed downstream of the light collimation unit, seen in the direction of light propagation, where the enlarging unit includes a transmissive volume hologram realizing an anamorphic broadening of the light wave field due to a transmissive interaction of the light wave field with the volume hologram, and the light modulator is disposed upstream or downstream of the anamorphic enlarging unit, seen in the direction of light propagation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/808,379, filed Mar. 18, 2013, which claims the priority ofPCT/EP2011/055593, filed on Apr. 11, 2011, which claims priority toGerman application No. 10 2010 031 024.7, filed Jul. 6, 2010; GermanApplication No. 10 2010 043 191.5, filed Oct. 29, 2010, andInternational Application No. PCT/EP2011/054660, filed Mar. 25, 2011,the entire contents of all of which are hereby incorporated in total byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a holographic display with anillumination device, an enlarging unit and a light modulator. Theholographic display serves to present two-dimensional and/orthree-dimensional image information.

There are two major problems one faces when realising holographicdisplays with large screen size:

-   -   If a large-area light modulator, e.g. having a diagonal        measurement of 24″, is used to encode the hologram, then this        large area of the light modulator must be illuminated uniformly        with sufficiently coherent light.    -   In contrast, if a small light modulator is combined with a        projection arrangement, then the device will be deeper than 1 m        if it has the same screen size of 24″ and if conventional        optical means such as lenses and mirrors are used.

It should be possible to solve the first problem with a largeillumination device that is as flat as possible. The second problem canonly be solved if other means than conventional optical means are usedto enlarge the illumination device and/or the light modulator.

A holographic projection display is disclosed for example in document WO2006/119760 A2. In that document, a light modulator with a small surfaceand high resolution, on which a hologram is encoded, is imaged in anenlarged manner with the help of an arrangement of lenses and mirrorsonto a lens or concave mirror which serves as a screen and reconstructedin a space which stretches between the screen and a viewing window whichis situated in the Fourier plane of the screen. Thanks to the enlargedimaging of the hologram onto the screen, that arrangement has theadvantage that the reconstruction space is enlarged too, so that muchlarger objects can be reconstructed than in conventional holographicarrangements. However, this goes along with the disadvantage that theoptical arrangement is rather voluminous and particularly long in theaxial direction, so that it can hardly be used as a holographic desktopdisplay because of its large depth.

In the projection display which is disclosed in document US 2007/252956A, a small light modulator is illuminated by a relatively smallillumination device and projected in an enlarged manner onto a screenwith the help of an extra-axially disposed holographic mirror element.It is an advantage of that arrangement that the axial dimension of theentire system is shortened because of the oblique optical path. However,the arrangement is still too voluminous to be used as a holographicdesktop display.

Document WO 2002/082168 A discloses a flat projection display whichcombines a one-dimensional and a two-dimensional grating for lightdeflection. The virtual image of a video projector is guided through arod-shaped grating body in one direction, and it is then guided througha plate-shaped grating body in a second direction which is perpendicularto the first one. In one embodiment, the gratings are made of glassstrips which are joined in layers at an angle of 45° to the surface ofthe display, each of which deflecting the light at right angles to thedirection of incidence. However, the image of the light modulator isthereby rather multiplied than enlarged, and an observer who looks atthe surface of the plate-shaped grating body in the normal directionsees a two-dimensional arrangement of one and the same modulator image.A holographic projection display where the encoding surface of the lightmodulator is actually enlarged cannot be realised with such anarrangement though.

In document WO 2002/31405 A, a collimated pencil of light rays withrectangular cross-section, which is for example emitted by a lightmodulator, is broadened in two perpendicular directions in that it fallsat a small angle on a one-dimensional surface that is not reflectinglike a mirror and on a two-dimensional surface that is not reflectinglike a mirror one after another. The two-dimensional broadening isachieved through the flat, “grazing” incidence, and the surfaces havesuch texture that they reflect the light beams into the desireddirection, that is perpendicular to the direction of incidence in thiscase. This is realised with the help of two-dimensional diffractiongratings or holographic surface gratings. In that arrangement, thecross-sectional area of the incident light wave field is truly enlarged,but there is no mention of defined amplitude and phase modulation of thepencils of light rays when they are reflected by the diffractiongratings, which would, however, be essential in the context ofholographic reconstruction of three-dimensional scenes.

SUMMARY OF THE INVENTION

It is thus the object of the present invention to provide a holographicdisplay with an illumination device which has an area that is as largeas possible while having a depth that is as little as possible and whichinvolves a minimum number of primary light sources only. It is a furtherobject of the present invention to enlarge a holographically encodedlight modulator that is as small as possible to a sufficiently largesize without thereby considerably increasing the depth of thearrangement. In either case, the angular spectrum of plane waves emittedby the illumination device and their coherence properties shall satisfythe requirements of a holographic or mixed holographic and stereoscopicrepresentation of objects.

These objects are solved according to this invention by the features ofclaim 1. Further preferred embodiments and continuations of the presentinvention are defined in the dependent claims.

The holographic display according to this invention comprises anillumination device, an enlarging unit and a light modulator. Theillumination device comprises at least one light source and a lightcollimation unit. The light collimation unit is designed such that itcollimates the light that is emitted by the at least one light sourceand generates a light wave field of the light that is emitted by thelight source with a specifiable angular spectrum of plane waves. Theenlarging unit is disposed downstream of the light collimation unit,seen in the direction of light propagation. The enlarging unit comprisesa transmissive volume hologram which is designed and disposed such thatan anamorphic broadening of the light wave field can be realised due toa transmissive interaction of the light wave field with the volumehologram (VH). Here, the light collimation unit and the enlarging unitcan preferably be used each for itself in a holographic or stereoscopicor autostereoscopic display. The light collimation unit and theenlarging unit—then as a collimating and enlarging module—can provide abroadened collimated light wave field for such a display according tothis invention. In the context of the present invention, anamorphicbroadening means in particular a beam broadening or enlargement of anincident light beam or light wave field without the provision of anintermediate optical image.

The light modulator for encoding the hologram information can bedisposed upstream or downstream of the anamorphic enlarging unit, seenin the direction of light propagation.

The light wave field coming from the light collimation unit can hit thevolume hologram at a specifiable angle of incidence, which should not besmaller than 70°. The angle of incidence here relates to the surfacenormal of the volume hologram and takes into account a possibledifference in the refractive indices of the optical media in front ofand behind the volume hologram.

The thickness of the volume hologram is chosen such that the light wavefield shows an angular distribution of wave vectors and that the maximumdeviation of the angle distribution of wave vectors of the light wavefield does not exceed a value of 1/20° in at least one direction. Thewave vector describes the direction of propagation of the waves of thelight wave field.

The difference in optical path length of the light beams of the enlargedlight wave field between two defined points on the light modulator shallnot exceed a given value on the encoding surface of the light modulatorat a given coherence length of the light. This means that the differencein optical path length between two arbitrary light beams which passthrough a given sub-region of the light modulator, which can for examplecorrespond with a sub-hologram, shall be small enough that these lightbeams are still capable of generating interference. A definition of asub-hologram is given in document WO 2006/066919 A1. Insofar, it is madesure with a thus defined coherence length of the used light thatconstructive and destructive interference is still possible in a displaybased on document WO 2006/066919 A1, so that three-dimensional scenescan be presented holographically to an observer with the display.

The enlarging unit can comprise a further volume hologram disposed moredownstream in the direction of light propagation, where the volumeholograms of the enlarging unit are designed and disposed such that thelight can be deflected into two different directions, where the lightmodulator is disposed upstream or downstream of the further volumehologram, seen in the direction of light propagation. According to thisembodiment, the first volume hologram serves to broaden or enlarge in afirst direction the light wave field which is collimated by the lightcollimation unit. The further (or second) volume hologram, which isdisposed downstream of the first volume hologram, serves to broaden orenlarge in a second direction the light wave field which has beenenlarged in the first direction by the first volume hologram. Thereby,for example just one primary light source can preferably illuminate alarge area or region substantially homogeneously, where the enlargingunit preferably takes very little space.

The two volume holograms can be arranged such that they broaden thelight wave field with the defined angular spectrum of plane wavesanamorphically in two substantially perpendicular directions, i.e. witha different enlargement factor in each direction.

The used volume holograms are preferably off-axis volume holograms,where object beam and reference beam do not lie on the same axis.

A laser, a laser diode, LED or OLED can serve as the light source.

The radiation or light of multiple light sources can be combined andinjected into a common optical fibre by a beam combiner. If only asingle light source is used, then its light can be guided to the lightcollimation unit through an optical fibre.

A primary collimation lens which serves to generate a collimated lightwave field can be disposed downstream of the point of light exit out ofthe optical fibre. This collimated light wave field can for example beused to illuminate a stereoscopic display.

Further, the primary collimation lens can be followed in the directionof light propagation by an angular filter in the form of a volumehologram, whose thickness is chosen such that the light wave field showsan angular distribution of wave vectors and that the maximum deviationof the angle distribution of wave vectors of the light wave field doesnot exceed a specifiable value of for example 1/20° in at least onedirection. This makes it possible to limit the angular spectrum of planewaves in at least one direction to a specifiable angular range alreadyin the light collimation unit and to define the thickness of the volumeholograms which are disposed downstream of the light collimation unitonly under consideration of the desired beam broadening or beamdeflection effect.

The collimated light wave field can illuminate a first micro-lens arrayof the light collimation unit.

A scattering device can be disposed in the focal plane of the firstmicro-lens array, from which the light propagates to a first aperturestop which is situated immediately downstream of the scattering device,where the apertures of the first aperture stop can have asymmetriclateral extents in order to generate an angular spectrum of plane wavesof the light wave field with specifiable coherence properties withregard to the respective lateral extent. This is particularly importantin the case of a mixed holographic and stereoscopic encoding of thedisplay, where the light wave field must exhibit sufficient coherence inthe direction of the holographic encoding, but sufficient incoherence inthe direction of the stereoscopic encoding.

The apertures of the first aperture stop of the light collimation unitare dimensioned such that the coherence properties of the light wavefield differ in two different directions such that the radiation issubstantially incoherent in the one direction, whereas it issufficiently coherent in the other. Generally, the degree of coherenceof the radiation is the larger the smaller the aperture in therespective direction.

A second micro-lens array is preferably arranged downstream of the firstaperture stop in the direction of light propagation such that theapertures of the first aperture stop coincide with the rear focal pointsof the corresponding micro-lenses. The second micro-lens array thusgenerates a segmented light wave field with an angular spectrum of planewaves with which a following light modulator which carries a holographiccode is illuminated either directly or after lateral enlargement of thelight wave field.

Two further aperture stops are preferably disposed between the firstaperture stop and the second micro-lens array, said further aperturestops serving to prevent light of a secondary light source of the firstaperture stop from propagating to a different micro-lens than the one itis assigned to (illumination cross-talking).

The light modulator can be of a transmissive, reflective ortransflective type.

The illumination device is designed and dimensioned such that itilluminates the active area of the light modulator substantiallyhomogeneously.

When the light waves are diffracted by the volume holograms, however,the angular spectrum of the light wave field is modified such that forexample the modification of the angular spectrum of plane waves of thelight collimation unit when being diffracted by the volume hologramsmust be taken into consideration when choosing the parameters of thelight collimation unit. For example, the anamorphic broadening by afactor of 10 will cause the angular spectrum of plane waves to bereduced on average by the same factor in that direction. It can thus benecessary that at least one parameter of the light collimation unit ismodifiable in order to generate a specifiable angular spectrum of planewaves of the light wave field downstream of the at least one volumehologram. This can be realised for example by way of a controlled ormanual adjustment of a respective optical component of the lightcollimation unit or by adequate design of the light collimation unit fora specific application.

However, it is also possible at the same time that the angular filteringeffect of the at least one volume hologram is used to suppressdisturbing portions of radiation or diffraction orders for an observerwho looks at the display. This is particularly useful in a holographicdisplay as described in document WO 2006/066919 A1, because higher orunwanted diffraction orders must be suppressed or blanked out there.

Further, there is the possibility that one of the volume holograms isdesigned such that it has the function of a field lens, in addition toits function of a broadening element. Thanks to such a field lensfunction, a real or virtual light source can be imaged into an imageplane of the light source in a holographic display as described indocument WO 2006/066919 A1.

The present invention is particularly preferably applied in aholographic display as described in documents WO 2006/066919 A1 or WO2004/044659 A2. It allows to give the holographic display a flat andspace-saving design.

With very high frame rates of for example ≥240 fps (frames per second),it is advantageous to design the illumination device such thatindividual regional segments can be turned on and temporally modulatedindependently of each other, so that for example only those regions onthe light modulator are illuminated which have reached the desiredadjustment value or set-point value (e.g. the phase plateau during theswitching operation of a liquid crystal phase modulator). Thisoperational mode is also referred to as scanning.

For this, it makes sense to modify the illumination device of theholographic display such that a shutter is disposed upstream of thefirst micro-lens array of the light collimation unit (in the directionof light propagation), where multiple strip-shaped segments which run inthe horizontal or vertical direction can be activated in said shutter,i.e. such that strip-shaped regions on a subsequently disposed lightmodulator can be illuminated optionally.

One realisation option of a scanning illumination of the light modulatoris then for example that always two strip-shaped segments of theilluminating light are switched on which run vertically in the plane ofthe light modulator and which can be moved sequentially in thehorizontal or vertical direction between the edge of the light modulatorand its centre.

However, the use of shutters for light control has the disadvantage thatit comes with a loss in light output, because only a small portion ofthe shutter elements are switched on, i.e. transparent at any one time.

Another possibility of realising large-area scanning illuminationdevices is to not enlarge the segmented plane wave field which isemitted by a miniature plane light collimation unit by a combination oftwo volume gratings in two perpendicular directions, but rather to usethe second volume grating only, which has the two-dimensional enlargingeffect, and to illuminate it by a light collimation unit with linestructure, where a line has at least two light sources which can beswitched independently of each other and, at the exit, at least twocollimating refractive lenses, and where these lines are arranged sideby side along an edge of the subsequently arranged two-dimensionalenlarging unit such that they illuminate the entire surface of thelatter. The volume grating diffracts the light beams which are incidentat a flat angle such they leave the volume grating substantiallyperpendicular to its surface.

The illuminating surface which is formed by the exit of the line-shapedlight collimation unit can also illuminate the entry surface of awedge-shaped light waveguide device made of a refractive material suchas glass to whose exit surface, which is substantially perpendicular toits entry surface, the two-dimensional volume grating is attached.

It is also possible that instead of the wedge-shaped light waveguidedevice made of a refractive material no optical medium or air isprovided and that the illuminating light that is emitted by theline-shaped light collimation unit falls directly onto thetwo-dimensional volume grating or a material that carries thetwo-dimensional volume grating.

This arrangement enlarges the segmented plane wave field which isemitted by the light collimation unit and directs it at the surface of afollowing light modulator.

Depending on the number of lines which are arranged side by side in thelight collimation unit and the number of light sources which can beswitched independently of each other in each line, the thus formedillumination device has a matrix of independently switchableillumination segments.

However, this solution is rather inefficient if each segment of the thusformed illumination device is illuminated and switched by a dedicatedlight source, as is described for example in document WO 2004/109380.

In order to improve utilisation of the available light power and,moreover, to do with as few primary light sources as possible, it makessense to control and distribute the light of very few light sources e.g.through a system of cascading light waveguides or switches.

An active optical switch can for example redirect the light from oneoptical fibre to another one when a voltage is applied. If multiple ofsuch branches are connected in line, for example in a tree structure,then a single primary light source can generate 2 to the power of Nswitchable secondary light sources, where N is the number of cascades.

One realisation option in this respect can thus be such that selectedlenses of a primary collimation lens array which is situated upstream ofthe first micro-lens array of the light collimation unit are illuminatedby such a cascade of switchable fibre-optic light waveguides.

A further possibility of illuminating selected lenses of a primarycollimation lens array which is situated upstream of the firstmicro-lens array of the light collimation unit is to provide passivelight exit points at the ends of optical multi-mode fibres whichilluminate one or more primary collimation lenses, according to theiractual arrangement.

However, this option means that the light of one primary light source isdistributed to multiple secondary light sources without the possibilityof actively controlling the individual secondary light sources.

Besides fibre-optic light waveguides and switches, light deflectingelements such as liquid crystal gratings can be used as well in order toilluminate selected segments of the first micro-lens array of the lightcollimation unit using a combination of two switchable LC-baseddiffraction gratings which are disposed between a primary collimationlens, which is disposed downstream of the light source, and the firstmicro-lens array of the light collimation unit, where the intensity ofthe for example strip-shaped segments can also be varied locally.

The illuminating regions which are generated by combinations ofswitchable diffraction gratings can also directly illuminate the entrysurface of an enlarging unit based on volume gratings and be enlarged bythem. The main advantage is that there is no need for a light-absorbingshutter.

Such a combination of gratings can for example comprise a firstdiffraction grating whose deflection angle can be controlled through thegrating constant, whereby a light beam which hits the surface at a rightangle leaves the diffraction grating at a certain angle, and a second,controllable diffraction grating which deflects and directs this lightbeam such that it leaves the grating surface substantially at a rightangle again. The amount of the lateral offset of the light beam is thendefined by the deflection angle and the distance between the twodiffraction gratings.

Since the scanning steps are generally discrete, PDLC volume gratings orpolarisation gratings can be used alternatively to LC gratings asswitchable diffraction gratings for light deflection and combined withswitchable retardation plates. The switchable retardation plates servefor actively switching the polarisation of the light beams. It is thusalso possible for example to use a set of polarisation-switchingpolarisation gratings where the gratings show the same intensity in thepositive and negative first diffraction order.

For a specifiable series of fix scanning steps, it is also possible touse an angle division multiplex in conjunction with angle-sensitivevolume gratings, where the first diffraction grating is of a switchabletype and where the second diffraction grating is disposed upstream ofthe first micro-lens array of the light collimation unit and designed inthe form of an angle-sensitive volume grating through which thespecifiable deflection angles for at least one light wavelength arerealised thanks to a firmly inscribed diffractive structure. The firstone of the two gratings can for example also be a switchable PDLCgrating stack.

Now, while the first grating or grating stack realises the activeangular deflection of the incident light beams which are collimated bythe primary collimation lens, the light beams are laterally offset andoriented parallel to the optical axis by the passive angle-sensitivevolume grating depending on their angle of incidence.

Light can also exclusively be deflected by way of space divisionmultiplexing, where the first diffraction grating is of a switchabletype and where the second diffraction grating is disposed upstream ofthe first micro-lens array of the light collimation unit and designed inthe form of a volume grating which comprises multiple strip-shapedsegments, and where the strip-shaped segments are made such that thelight which hits them at an angle that increases as the distance to theoptical axis becomes larger is diffracted into a direction that isparallel to the optical axis. This means that with this option therespective strip-shaped segment of the second grating realises a firmlyinscribed deflection angle in order to orient a light beam which isincident at an angle to the optical axis such that is becomes parallelto the latter again. This grating can for example also have a region inits centre where no volume grating is inscribed at all, so that thedirection of propagation of an incident light beam is not affected.

Besides optical paths which run parallel to the optical axis of thearrangement, it is also possible to realise optical paths which run atan angle to it or asymmetrically. This requires the diffraction gratingswhich are disposed between the primary collimation lens of the lightsource and the first micro-lens array of the light collimation unit tobe designed such that off-axis optical paths can be realised as well,for example in order to eliminate the 0th diffraction order of thegratings if only the first or higher diffraction orders shall be usedfurther down the optical path.

The diffraction gratings which are disposed between the primarycollimation lens of the light source and the first micro-lens array ofthe light collimation unit can also be designed such that certainregions on the first micro-lens array of the light collimation unit canbe illuminated in a switchable way in the horizontal and/or verticaldirection. This way, an illumination that is oriented in two differentdirections, i.e. a two-dimensionally scanning illumination, of thesubsequent light modulator can thus be realised.

There will be a special problem if the first micro-lens array of thelight collimation unit is not illuminated by a single light sourcecombined with a large-area collimation lens, but rather by multiplelight sources combined with a collimation lens array. The problem hereis a broadened angular spectrum of plane waves of the illuminationcaused by the diffraction at the edges of the lenses, and it requiresadditional measures for compensation.

The lenses of the first micro-lens array of the light collimation unitare for example illuminated by segmented plane waves whose angularspectrum exhibits an angular deviation of about 1/20° in one directionand of about 1° in the perpendicular direction. This limitation of theangular spectrum of plane waves can be necessary e.g. in a holographicdisplay which takes advantage of a one-dimensional horizontal orvertical holographic encoding method.

One solution is then for example to realise an additional angularfiltering of the wave field, where for preventing the broadening of theangular spectrum of plane waves through diffraction at the edges of thelenses of the collimation lens array it is followed in the direction oflight propagation by a combination of two volume gratings for angularfiltering.

The combination of volume gratings for angular filtering of the angularspectrum of plane waves comprises a first, thin volume grating with awide angular selectivity and a large diffraction angle deviating fromthe grating surface normal and a second, thick volume grating with anarrow angular selectivity which is designed such that the light beamswhich are incident in the region of the given angular spectrum of planewaves are substantially diffracted along the grating surface normal andthat the light beams which propagate outside the angular spectrum ofplane waves are transmitted without diffraction.

An illumination device which has an angular spectrum of plane waveswhich is limited to ≤1/20° at least in one direction and which comprisesa multitude of light sources and a collimating lens array can be createdthis way for a direct-view display.

To be able to realise an angular filtering of the angular spectrum ofplane waves in two perpendicular directions, a second combination ofvolume gratings, which is turned by 90° relative to the first one, canbe disposed downstream of the first combination of volume gratings.

The illumination devices for transmissive light modulators (backlightunits BLU) can generally also be modified such to illuminate reflectivelight modulators (frontlight units FLU).

It is for example possible to supplement an illumination device with alarge-area volume grating as enlarging unit by a retardation plate, inparticular a λ/4 plate, which is disposed downstream of thetwo-dimensional volume grating in the direction of light propagation. Iffor example horizontal linear polarised light falls on this λ/4 plate,then it will leave the plate having a circular polarisation. Areflective light modulator which is disposed downstream in the opticalpath reflects the modulated circular polarised light back towards theλ/4 plate; after having passed through this plate again, it exhibitsvertical polarisation. This vertical polarised light can now passthrough the volume grating unimpeded and without interfering with theinitially horizontal polarised light, and it can be perceived by anobserver who is situated in front of the volume grating.

Besides the polarisation-wise separation of the illuminating light onthe one hand and the reflected and modulated light on the other, thereis another way of separating them, namely to take advantage of theangular selectivity of a light deflecting element, such as a volumediffraction grating, which injects the light which is emitted bysuitable light sources into a plane waveguide which covers the entiresurface of the light modulator and which also couples out the light inorder to illuminate the light modulator.

For example, if a transmissive volume grating of sufficient thickness isused and if the light modulator is illuminated at a sufficiently obliqueangle, i.e. for example 5°, then there is an ‘off-Bragg’ illumination ofthe volume grating on the way back of the modulated light from thereflective light modulator, and this volume grating which is used toilluminate the light modulator thus has no diffracting function. Thelight which is reflected and modulated by the light modulator can thuspropagate to the observer without being obstructed.

BRIEF DESCRIPTION OF THE DRAWINGS

Now, there are a number of possibilities for embodying and continuingthe teachings of the present invention. To this end, reference is madeon the one hand to the dependent claims that follow claim 1, and on theother hand to the description of the preferred embodiments of thisinvention below including the accompanying drawings. Generally preferredphysical forms and continuations of the teaching will be explained inconjunction with the description of the preferred embodiments of theinvention and the accompanying drawings. The Figures are schematicdrawings, where

FIG. 1 shows an illumination device comprising a light collimation unitin front of two transmissive volume gratings which broaden the wavefield in two directions one after another,

FIG. 2 is a side view of the light collimation unit of FIG. 1,

FIG. 3 illustrates the principle of the twofold beam broadening in anillumination device in two perpendicular directions with the help of twotransmissive volume gratings,

FIG. 4 shows the twofold beam broadening by a factor of 10 each asrealised by the embodiment illustrated in FIG. 3,

FIG. 5 shows an anamorphic enlargement of a light modulator (SLM, left)by a factor of 10 in the horizontal direction in the plane of aholographic off-axis lens,

FIG. 6 is a top view which shows an arrangement with a reflective lightmodulator (SLM, bottom, disposed on a base plate) which isanamorphically enlarged by a factor of 10 in one direction with the helpof an off-axis field lens which is designed in the form of atransmissive volume grating,

FIG. 7 shows the enlargement of the encoding surface of a lightmodulator where the difference in optical path length of the light beamsat the various points on the encoding surface after passage through thearrangement is indicated by different shades of grey,

FIG. 8 shows the design of a single cell of a line-shaped lightcollimation unit,

FIG. 9 illustrates a further embodiment of a flat illumination devicewith a light collimation unit comprising a double row of collimatingrefractive lenses (left: side view, right: perspective view showingthree double lenses only),

FIG. 10 illustrates a further embodiment of a flat illumination devicewith a light collimation unit comprising a double row of collimatingrefractive lenses (left: side view, right: front view showing two linesof the light collimation unit only),

FIG. 11 shows a further embodiment of a scanning illumination devicewith a shutter which is segmented in stripes, said device realising ananamorphic enlargement of the wave field which occurs downstream of thelight collimation unit,

FIG. 12 shows an active optical switch for switching the light emittedby a primary light source between two optical fibres,

FIG. 13 shows an embodiment of a light collimation unit which makes itpossible to illuminate selected lenses of a collimation lens array withthe help of a cascade of optical fibre switches,

FIG. 14 shows a passive light exit point at the end of a multimodefibre,

FIG. 15 shows a further embodiment of a light collimation unit whichmakes it possible to illuminate selected regions of a collimation lensarray with the help of two LC gratings,

FIG. 16 shows an embodiment of a scanning illumination device with alight collimation unit which makes it possible to directly illuminatethe entry surface of a subsequently arranged enlarging unit according toFIG. 11 with strip-shaped illumination regions with the help of two LCgrating according to FIG. 15,

FIG. 17 illustrates the angular filtering effect of a combination of twovolume gratings VG1 and VG2,

FIG. 18a shows a further embodiment of a light collimation unit whichmakes it possible to illuminate selected parabolic mirrors of acollimation parabolic mirror array of an illumination device forreflective light modulators with the help of optical fibre switches,

FIG. 18b shows embodiments of line-shaped light collimation units forinjecting the light through volume gratings into plane waveguidesaccording to FIG. 18 a.

Identical or comparable parts are given like reference symbols in allthe Figures.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of an illumination device of the holographicdisplay, said illumination device comprising a light collimation unit infront of two transmissive volume gratings which broaden the light wavefield in two different directions one after another. Here, the lightcollimation unit which comprises two micro-lens arrays preferably has asmall size.

The light wave field is broadened anamorphically, i.e. the enlargementfactor differs in the two different directions.

The light which is emitted by a power-(P)-and-wavelength-(λ)-stabilisedlaser diode sLD is coupled into an optical fibre OF through agradient-index lens GRINCL.

The divergent light which is emitted by the end of the optical fibre iscollimated by the light collimation unit, i.e. formed into a plane wave,which means that the rays of light are oriented in parallel through thiscollimation. The light collimation unit comprises a primary collimationlens pCL.

The first micro-lens array fML of the light collimation unit LCU focusesthe light which falls on this micro-lens array fML in the focal plane ofthe individual micro-lenses and thus generates an array of secondarylight sources sLS.

A scattering plate sPS, which is disposed in the focal plane of themicro-lenses of the first micro-lens array fML, allows the phase of thelight to be scattered statistically in space. This scattering plane sPS(see FIG. 1) can for example be a scattering plate sPS which is movedmechanically (e.g. by one or more piezo crystals).

The statistic, temporally variable spatial phase modulation in thesecondary light source plane is necessary to be able to generate anilluminated area (called a sweet spot) in the incoherent direction on alight modulator if the hologram is encoded one-dimensionally.

An aperture stop AS(sLS) is disposed downstream of the plane of thescattering plate sPS and serves to limit the spatial extent of thesecondary light sources sLS. If a one-dimensional encoding method isused, sufficient spatial coherence must be ensured in one direction.This is achieved by controlling the size of the statisticallyphase-fluctuating light source. The second, coherent direction ischaracterised by a small spatial extent of the secondary light source.The apertures of the aperture stop AS(sLS) are thus extremelyasymmetrically, for example 15 μm in the incoherent direction and 0.5 μmin the coherent direction, in order to create an angular spectrum ofplane waves in an angular range of 0.5° and 1/60 downstream of thesecond, collimating micro-lens array cML.

Two aperture stops aAS1 and aAS2 are disposed between the aperture stopAS(sLS), which serves as an array of secondary light sources, and themicro-lens array cML, which collimates the secondary light sources sLS,and are used to prevent illumination cross-talking, i.e. to preventlight of a secondary light source from reaching adjacent micro-lenses,i.e. other micro-lenses than the ones they are assigned to.

FIG. 2 shows the light collimation unit LCU of FIG. 1 in a side viewfrom the left. The primary light source here has 3 laser diodes R, G, B,representing the colours RGB, whose red, green and blue radiation iscombined in an optical fibre OF.

In FIG. 2 the reference symbols denote the following elements: R: redlaser diode; G: green laser diode; B: blue laser diode; pLS: primarylight source; YJ1 and YJ2: Y junctions 1 and 2; OF: optical fibre;cpLS(RGB): combined primary light source (red, green, blue); pCL:primary collimation lens; cWF: collimated wave front; fMLA: focussingmicro-lens array); sPS+PZT: statistic phase scattering and piezotranslation element; AS(sLS): aperture stop (defines the active area ofthe secondary light sources); AS(ict)1+2: aperture stops 1 and 2 toavoid illumination cross-talking; cMLA: collimating micro-lens array;scWF: segmented collimated wave front.

FIG. 3 illustrates the principle of the twofold beam broadening in anillumination device in two different (here perpendicular) directionswith the help of two transmissive gratings in the form of volumegratings. The light wave field which comes from the LCU is deflected andbroadened by the first transmissive volume hologram VH1. Thereafter,this light wave field is deflected and broadened once again by thesecond transmissive volume hologram VH2.

In the embodiment shown in FIG. 1 the light collimation unit is disposedupstream of these two gratings in the optical path SG.

The volume holograms of the enlarging unit can preferably bemanufactured for example by way of in-situ exposure of accordinglysensitised light-sensitive materials having a suitable thickness. Thisway, the real aberrations which are existing in the illumination devicecan be compensated by these volume holograms.

FIG. 4 illustrates how the wave field of a segmented collimated wavefront is broadened in two directions one after another by a factor of 10each with the help of two transmissive volume gratings VH1 and VH2 whichare disposed downstream of the light collimation unit, as shown in FIG.3.

The angular spectrum of plane waves of the segmented collimated wavefront scWF which exists downstream of the light collimation unit LCU ismodified by way of diffraction in the two volume holograms or gratings.The angle θ_(S) of the individual diffraction orders m of the signalwaves downstream of the grating is calculated as follows:θ_(S)=arcsin(mΛ/(nΛ _(x))+sin(θ_(R)))  (Equation 1)

where Λ is the wavelength, n is the refractive index, Λ_(X) is theperiod at the surface of the volume grating, and θ_(R) is the angle ofthe reconstructed beam, i.e. the angle at which the illuminating beamhits the volume grating in rad. The sign convention of the angles forquadrants 1, 2, 3 and 4 is +, +, − and −.

The arcsin(x) is derived as follows:

$\begin{matrix}{{\frac{d}{dx}{\arcsin(x)}} = \frac{1}{\sqrt{( {1 - x^{2}} )}}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

Then, dθS/dθ_(R) is:

$\begin{matrix}{\frac{d\;\theta_{S}}{d\;\theta_{R}} = \frac{\cos( \theta_{R} )}{\sqrt{( {1 - ( {{m\;{\Lambda/( {n\;\Lambda_{x}} )}} + {\sin( \theta_{R} )}} )^{2}} )}}} & ( {{Equation}\mspace{14mu} 3} )\end{matrix}$

The target angular ranges of the angular spectrum of plane waves PWS is1/60° in the coherent direction and 0.5° in the incoherent direction.Assuming that) tan(0.5°)*1000 mm=8.73 mm, it can be said that ±0.25°angular spectrum of plane waves is sufficient to create a 9-mm widesweet spot at a distance of 1 m to the display. The angular spectrum ofplane waves of the illumination should not be chosen too wide, becauseit can still be broadened further by a deflection unit for observertracking (see document WO 2006/066919 A1, for example) that is arrangeddownstream of the display as such. Such a deflection unit is describedfor example in documents WO 2010/066700 or PCT/EP2010/058625.

If deflection angles for observer tracking are 30° and more, the angularspectrum of plane waves which exists for example upstream of thedeflection unit can be chosen smaller than 1/60° in the coherentdirection too, for example only 1/100°, so to ensure that the angularresolving power limit of the human eye (which is about 1/60°) is notexceeded even at large deflection angles.

However, according to Equation (3), there is an angle reduction by thefactor of 0.1 in the volume gratings which are shown in FIGS. 3 and 1.This means that if there is an angular spectrum of plane waves of ±0.25°at θ_(R0)=84.26° and θ_(S0)=0°, i.e. in the design geometry, then itwill be transformed to an angular spectrum of plane waves of ±0.025°downstream of the grating. If the geometry is the other way round, thenthe factor will be 10.

The angular spectrum of plane waves of the light collimation unit isthus ±1/12° and ±2.5° for a one-dimensional encoding of the lightmodulator. With this type of encoding, the three-dimensional scene isencoded or generated holographically in the one direction andstereoscopically in the perpendicular direction. Depending of thedirection of holographic encoding, it is referred to as a horizontalparallax only (HPO) or vertical parallax only (VPO) type encoding.

In the event of an exclusively stereoscopic encoding, where thecoherence properties of the illumination do not play a role, alimitation of the angular spectrum of plane waves to a certain angularrange which is much smaller than 1° is irrelevant, so that an angularrange of no more than 3° is definitely permissible in the horizontaland/or vertical direction.

As shown in further embodiments, the explanations above can also applyto achieve an enlargement of the encoding surface of the light modulatorof a display, in particular of a holographic display. Here, it ispreferably possible to minimise the number of optical components used inthe holographic display. In addition, it is further preferable tominimise the size of the light modulator at least in one direction (seeFIG. 5).

For this, it is for example possible to use a volume grating which isdesigned in the form of an off-axis field lens in order to achieve ananamorphic enlargement preferably in the incoherent direction of thelight modulator—for example of a one-dimensionally encoded holographicdisplay.

The anamorphic enlargement in one direction has the advantage that areflection-type light modulator can be used which is as high as thedisplay but which has only 1/10 of the width of the display. This isshown in FIG. 6.

The fact that the enlargement is achieved with the help of an off-axislens, which is realised in the form of a transmissive volume grating,reduces the number of components in the display. The lens can have theeffect of an angular filter in one direction. This means that the lightmodulator can be attached to the bottom edge of the display, where thevolume grating can cut the angle of the virtual viewing window (e.g. theviewing window VW in document WO 2006/066919 A1) in the coherentdirection out of the wave front that impinges on the grating anddiffract only that angle towards the observer in its function of a fieldlens. This means that the view shown in FIG. 6 can also be the side viewof a holographic display.

The light collimation unit is shown in a simplified manner in FIG. 6. Itcan for example comprise two micro-lens arrays in addition to the shownprimary light source pLS and the primary collimation lens pCL, in orderto generate the segmented plane wave field which is required toilluminate the light modulator SLM with the help of apertures and ascattering plate (cf. FIG. 2, for example).

A deflection unit (not shown) for tracking the wave front to a movingobserver eye (observer tracking) may be disposed downstream of the fieldlens VH2, which is the key component to ensuring a compact and flatdesign. This can for example be two crossed variably controllablediffraction gratings, for example as described in documentPCT/EP2010/058625, which realise locally different deflection angles.

A compact design of an illumination device of a holographic display HDis shown in FIG. 1. As a consequence, the size of the light collimationunit LCU, which comprises two micro-lens arrays fMLA and cMLA, is verysmall. The embodiment shown in FIG. 1 was originally intended toilluminate a light modulator (not shown) which has substantially thesame size or outer dimensions as the display and which is disposeddownstream of the enlarging unit VE. However, according to a furtherembodiment, it can also be used to enlarge the light modulatoranamorphically. This is done in analogy with the design of a telescope,which effects for example a beam broadening by a factor of 10. Thevoluminous design of a telescope can thus be minimised greatly. This canbe achieved by way of consequently following the approach of beambroadening using gratings, which are here designed exclusively in theform of transmissive volume holograms VH1, VH2, which shall, however,not be construed to limit the universality of the invention. Theprinciple is shown in FIG. 3.

The light modulator can be disposed upstream of the enlarging unit VE ofFIG. 3 and be either of a reflective or of a transmissive type. However,reflective arrangements are preferred. Such a light modulator could forexample be designed in the form of a liquid crystal on silicon (LCoS)element or micro electro-mechanical system (MEMS), for example a digitalmicro-mirror device (DMD). A light collimation unit LCU and a lightmodulator can be provided for each of different colours or lightwavelengths, which can be combined with suitable beam combining elements(for example an X cube, comparable with beam splitter plates in colourCCD cameras, but run in the other direction) and coupled into theembodiment shown in FIG. 3.

To this end, in such an embodiment, it is not only the light collimationunit LCU that is disposed upstream of the two volume gratings VH1, VH2which serve as enlarging unit VE in the optical path SG, but the lightmodulator too is disposed upstream of the two volume gratings VH1, VH2according to this invention. The light modulator is then disposeddownstream of the light collimation unit LCU of FIG. 2, but upstream ofthe two volume gratings VH1, VH2 of FIG. 3.

The light modulator of a 24″ display with an aspect ratio of 16(horizontal):9 (vertical) then has a size of 53 mm×30 mm instead of 530mm×300 mm. Small light modulators can be run in the reflective mode.Because the response time τ˜d² (where d is the thickness of the SLM),operation in the reflective mode brings about a possible increase in theframe rate by a factor of 4. In addition, the electronic controlelements (backplane) do not have to be made transmissive.

FIG. 4 illustrates how the wave field of a segmented collimated wavefront is broadened or enlarged in two directions one after another withthe help of two transmissive volume gratings VH1 and VH2, which aredisposed downstream of the light collimation unit LCU, as shown inFIG. 1. This principle of a twofold beam broadening can not only betaken advantage of in order to be able to use a highly compact lightcollimation unit LCU in the display, but also a light modulator SLM ofvery small size, where the latter can also be of a reflective type.Light modulators with a small active surface are much less expensivethan those with a large active surface.

The two volume gratings VH1 and VH2, which are disposed downstream ofthe light modulator in the direction of light propagation, can be usedfor angular filtering in analogy with the illumination device (backlightunit) BLU. This means that the thickness of the volume gratings VH1, VH2shall be chosen such that the angular spectrum of plane waves is limitedto max.±1/20° in the coherent direction and to max.±1/2° in theincoherent direction. The enlarged light wave field sWF of the lightmodulator SLM can for example be arranged in space at an oblique angleto the second, two-dimensional volume grating VH2, depending on theactual design of the volume gratings VH1, VH2. In a holographic displayas described for example in document WO 2006/066919 A1, however,individual points of a three-dimensional scene to be represented can begenerated by way of holographic encoding in different depth regions.Insofar, when a three-dimensional scene is represented, a possibleinclination of a light wave field that is enlarged by the two volumegratings VH1, VH2 can be taken into consideration by creating theindividual points of the scene at accordingly different distances to thesecond volume grating VH2.

The inclination SLWF of a light wave field sWF that is enlarged by thetwo volume gratings VH1, VH2 is a result of the different optical pathlengths of the light beams when passing through the volume gratings VH1,VH2. This is shown in FIG. 7 for a light modulator SLM which is enlargedin two directions. The enlarged encoding surface of the light modulatorSLM shows differences in the optical path lengths of the light beamswhich pass though individual points, which is indicated by differentshades of grey in the front view (right-hand side in FIG. 7). Thisdifference is the greatest between the two diagonal corners with thegreatest brightness difference, i.e. bottom left and top right corner.This must be taken into account when encoding the depth of thethree-dimensional scene which is to be reconstructed by the display.

Another requirement which results from the difference in path lengthrelates to the coherence length of the light beams which are emitted bythe illumination device. Due to the difference in optical path length intwo individual points on the enlarged encoding surface of the lightmodulator, which can for example represent points of a sub-hologram (seedocument WO 2006/066919 A1), the coherence length of the light must begreater than the maximum possible optical path length difference betweenthese points, so that these light beams are still capable of generatinginterference. If the encoding surface is divided into sub-regions (asindicated in FIG. 7 in the form of squares), which can for examplecorrespond with sub-holograms, the coherence length must be greater thanthe difference in optical path length between the two diagonally opposedcorner points with the greatest difference in optical path length, sothat interference can still occur across the entire area of asub-hologram. As explained above, it must also be considered that thedifference in optical path length can be enlarged further bysubsequently arranged optical components, e.g. for observer tracking.

The surface area of a beam combining device, e.g. as described indocument PCT/EP 2010/058626, where it is referred to for example as‘light wave multiplexing means’, can preferably also be very small ifthe light modulators and the beam combining device are disposed upstreamof the enlarging unit VE. Alternatively, a birefringent calcite plate ofrelatively small dimensions can be used, which would serve to have asimilar effect.

Disturbing emission angles, e.g. as caused by diffraction at theapertures or cross-talking in the light collimation unit, can beprevented from propagating towards the observer eye thanks to theangular filtering function of the volume gratings VH1 and VH2. Theangular selectivity of the volume grating VH2 shall thus be chosen suchto suit the actual application.

The angular range of a virtual viewing window VW can be specifically cutout of the encoded wave field. This corresponds to a smoothening of theencoded wave function and can be optimised such that diffraction orderswhich occur beside the virtual viewing window VW are suppressed oravoided. The light modulator SLM should then be illuminated with anangular spectrum of plane waves of the light which does not exceed theangular range of 1/60° in the coherent direction. However, the angularrange can be as great as ±3° downstream of the light modulator SLM.

The illumination device according to the embodiments shown in FIGS. 1and 6 can for example also be designed in the form of and used as aso-called frontlight, which serves to illuminate a reflective lightmodulator. The polarisation of the light which is emitted by theillumination device and which falls on the light modulator can forexample be modified with the help of a retardation plate, so that thelight which is reflected by the light modulator can pass through theillumination device substantially without being deflected and propagatetowards the observer and that it does not re-enter the illuminationdevice. Such a retardation plate should be designed in a suitable mannerand it should be disposed between the illumination device and lightmodulator. As an alternative to using a retardation plate, theilluminating light can be prevented from re-entering the illuminationdevice after being reflected by the light modulator in that theilluminating light leaves the illumination device such that it will notbe reflected in itself when being reflected by the light modulator, forexample if the light leaves the illumination device at an angle of 5°relative to the surface normal of the light modulator. The volumegrating of the illumination device would have to be designed accordinglyfor this. In this case, the light which is reflected by the lightmodulator does not ‘see’ the volume grating of the illumination devicedue to a specifiable angular selectivity of the volume grating or volumehologram, thus passing through the illumination device substantiallywithout being deflected.

With very high frame rates of for example ≥240 fps (frames per second),it is advantageous to design the illumination device such thatindividual regional segments can be turned on and temporally modulatedindependently of each other, so that for example only those regions on asubsequently arranged light modulator are illuminated which have reachedthe desired adjustment value or set-point value (e.g. the phase plateauduring the LC switching operation).

One possibility of realising large-area scanning illumination devices isto not enlarge the segmented plane wave field which is emitted by aminiature light collimation unit by a combination of two volume gratingsin two perpendicular directions, but rather to use the second,two-dimensionally enlarging volume grating only, and to dispose alongone of its edges, namely the one from which the light falls on thegrating, in subsequent arrangement so many line-shaped light collimationunits comprising at least two light sources which can be switchedindependently of each other and, at the exit, at least two collimatingrefractive lenses that they illuminate the entire surface of the volumegrating across the entire width of the edge. After enlargement by thetwo-dimensional volume grating, an array of independently switchableillumination segments is created the total number of which is theproduct of the number of collimation lines and the number of switchablelight sources per line.

An individual line of such a light collimation unit is shown in FIG. 8,where the reference symbols have the following meanings: LS: lightsource; sPS: statistic phase scattering element; FL: focussing lens;AS(sLS): aperture stop (secondary light sources); B: base plate; aAS1:apodised aperture stop 1; aAS2: apodised aperture stop 2, CL:collimation lens.

The illuminating surface which is formed by the exit of the line-shapedlight collimation unit can also illuminate the entry surface of awedge-shaped light waveguide device made of a refractive material suchas glass to whose exit surface, which is substantially perpendicular toits entry surface, the two-dimensional volume grating is attached. Sucha light waveguide device is described by the embodiment according toFIG. 9 and denoted by the reference symbol LE.

It is also possible that instead of the wedge-shaped light waveguidedevice LE made of a refractive material no optical medium or air isprovided and that the illuminating light that is emitted by theline-shaped light collimation unit falls directly onto the plane volumegrating or a material that carries the two-dimensional volume grating.

The embodiment shown in FIG. 9 of a flat illumination device comprisinga double row of collimating refractive lenses is based on the feature tocontrol single light sources or output coupling points of lightwaveguides as secondary light sources. For example, 5-mm wide stripescan be illuminated independently of each other in the horizontaldirection. Each lens at the exit of the light collimation unit can forexample be assigned with a laser diode LD as a light source. If the twolaser diodes which are collimated by a double lens are switched on, thena vertical stripe with the width of a lens will be illuminated almosthomogeneously, e.g. the regions 11 and 12 in FIG. 10. However, theseregions can be switched on and off separately too.

The individual regions on the illumination device which can becontrolled, i.e. illuminated, independently of each other are numberedin FIG. 10. The illumination device is divided into two regionsvertically and into a multitude (e.g. 40 in FIG. 10) of regionshorizontally. The arrangement shown in FIG. 10 can also be viewed as oneof multiple sub-regions of a tiled illumination device. For example,there will be four segments vertically if two such sub-regions arejoined at their long ends. The gap width of the non-illuminated areahere is ≤100 μm so that it cannot be perceived by the observer if thedisplay plane or a plane in the immediate vicinity of the display plane,i.e. for example a plane in the depth of the display plane with adistance of between +10 mm and −20 mm is shown as a bright surface.

According to another embodiment of a scanning illumination device, ashutter that is segmented in stripes is disposed upstream of the firstmicro-lens array of the miniature light collimation unit of anillumination device (see FIG. 11) which works according to the principleof an anamorphic enlargement of the wave field downstream of the lightcollimation unit, where it is possible to control the transparency ofmultiple strip-shaped segments which run in the vertical or horizontaldirection. It is an advantage of this arrangement that disturbingdiffracted portions of the strip-shaped shutter are spatially filtered,i.e. blanked out, by the aperture stop (secondary light sources)AS(sLS).

With micro-lenses which have an aperture of for example 5 mm×5 mm, anadjustment tolerance of the segments of the strip-shaped shutter of Dx,Dy=0.1 mm is uncritical.

Depending on the scanning direction, the lens segments of thestrip-shaped shutter can be arranged horizontally in order to generatevertical stripes or vertically in order to generate horizontal stripes.

A preferred embodiment has two illuminated vertically or horizontallyrunning stripes lying in the display plane, i.e. in the plane of thelight modulator, where said stripes can be moved sequentially in thehorizontal or vertical direction between the edge of the light modulatorand its centre (see FIG. 11). The light source is turned on for example3 percent of the time.

The use of shutters goes along with a loss in laser power though. In theembodiment shown in FIG. 11, only 20 percent of the shutter surface aretransmissive. Moreover, if no wire grid polarisers WGP are used, thenthe transmittance will be less than 70 percent. This means that morethan 85 percent of the light is absorbed in the shutter plane.

The absorption loss can be minimised by using light waveguides inconjunction with fibre-optic switches. One possibility is to illuminateselected lenses of a primary collimation lens array which is disposedupstream of the first micro-lens array of the light collimation unit bya cascade of switchable fibre-optic light sources. It is for examplepossible that fibre switches as shown in FIG. 12 can switch 500 mW percolour variably between two exits.

FIG. 13 shows a cascade of fibre-optic switches foS in a lightcollimation unit which allows selected lenses of a collimation lensarray CLA to be illuminated. The collimation lens array CLA can comprisecylindrical lenses or lenses with square aperture. If cylindrical lensesare used, then the light source images must be broadened accordingly inone direction upstream of the CLA so to fully illuminate the cylindricallenses. Passive fibre light splitters—for example at a ratio of 1 to16—can be used for this. The arrangement shown in the Figure can also beapplied to a variable splitting of the light emitted by the primarylight source pLS into two planes.

FIG. 14 shows a passive light exit point at the end of an opticalmulti-mode fibre for illuminating specifiable lenses of a primarycollimation lens array that is disposed upstream of the first micro-lensarray of the light collimation unit. The lens L is situated for exampleupstream of the first micro-lens array fMLA of FIG. 13. Using thisarrangement, the length of the light collimation unit shown in FIG. 13can be reduced substantially.

The number of primary light sources should be kept as small as possiblebecause stabilisation of multiple lasers to have a common wavelength israther difficult. One possibility of generating a common wavelength isto use a coupled resonator. However, one primary light source per colouris the preferred embodiment.

Besides the use of fibre-optic switches, light-diffracting deflectionunits such as liquid crystal gratings can be used to illuminate selectedstripes of an illumination device in order to minimise the absorptionloss caused by shutters in scanning illumination devices. This isillustrated in FIG. 15, where selected segments of the first micro-lensarray of the light collimation unit can be illuminated by a combinationof two switchable LC-based diffraction gratings, which are disposedbetween a primary collimation lens that is situated downstream of thelight source and the first micro-lens array.

LC gratings also allow multiple stripes, i.e. more than two segments ofa collimation lens array, to be illuminated simultaneously. Moreover,the intensity can be varied locally within a stripe.

Since the scanning steps are discrete, switchable PDLC volume gratingscan be used as well to illuminate selected segments of an illuminationdevice in order to minimise absorption losses in scanning illuminationdevices.

Further, polarisation gratings combined with switchable retardationplates which turn the polarisation plane can be used as well. Stillfurther, it is possible for example to use a set ofpolarisation-switching polarisation gratings, where the gratings showthe same intensity in the positive and negative first diffraction order.

Still further, a minimisation of the absorption losses can be achievedby using angle division multiplex volume gratings. Since the scanningsteps are discrete and specifiable, angle division multiplexing can beused in conjunction with angle-selective volume gratings in order torealise a scanning illumination device.

The first diffraction grating of FIG. 15 is of a switchable type, andthe second diffraction grating, which is disposed upstream of the firstmicro-lens array of the light collimation unit, can for example beprovided in the form of an angle-selective volume grating, where thisangle-selective volume grating serves to realise the required deflectionangles for at least one light wavelength with the help of a firmlyinscribed diffractive structure.

The first diffraction grating of FIG. 15 can also be designed in theform of a switchable PDLC grating stack, where the second grating ofFIG. 15, which is disposed upstream of the first micro-lens array of thelight collimation unit, is designed in the form of a volume gratingwhich exhibits the necessary deflection geometries, which are designedsuch that the light which hits the grating at an angle that increases asthe distance of the arrangement to the optical axis becomes larger isdiffracted again into a direction that is parallel to the optical axis.

The diffraction geometry can be exclusively space division multiplexed.This means that the second grating of FIG. 15 can simply be a volumegrating which has for example ten spatially separated strip-shapedsegments with diffraction gratings having different optical properties,said diffraction gratings diffracting the light which impinges on themat an increasingly oblique angle as the distance to the optical axisbecomes larger to be parallel to the optical axis, depending on thewavelength RGB. This grating can for example also have a stripe in itscentre where no volume grating is inscribed at all, so that incidentlight is transmitted without being diffracted.

Besides optical paths in the light collimation unit which runsymmetrical to the optical axis, as is the case in the example shown inFIG. 15, oblique optical paths can be realised as well with a symmetryaxis that lies at an oblique angle to the optical axis of the lightcollimation unit. In such an off-axis arrangement, the intensity of the0^(th) diffraction order of the gratings used in that arrangement isuncritical, because it is guided away from the optical path that runsparallel to the optical axis.

The working principle which is illustrated for example in FIG. 15 can beextended to 2D scanning in that a second arrangement of gratings whichhas the same design but which is turned by 90° relative to the first oneis disposed downstream the first one. In addition, local dimming ispossible, in particular also with LC-based gratings, or with a lightcollimation unit that is extended to 2D scanning. Since fibre-opticswitches operate much faster than LC gratings, arrangements which usefibre-optic switches have greater response time reserves in applicationswhere scanning and local dimming are combined.

According to a preferred embodiment, the arrangement shown in FIG. 15,which allows selected regions of a collimation lens array of a lightcollimation unit which is disposed downstream to be illuminated with thehelp of two diffraction gratings G1 and G2, can also be used instead ofthe light collimation unit of the arrangement shown in FIG. 11 of ascanning illumination device with subsequent anamorphic enlargement ofthe wave field that occurs at the exit of the light collimation unit.This is shown in FIG. 16.

The major advantage of this embodiment of a scanning illumination deviceis its greater luminous efficacy, because no light-absorbing shutter isneeded to generate the strip-shaped illuminating regions. Referring toFIG. 16, the strip-shaped illuminating regions which are generated bythe two controllable diffraction gratings G1 and G2 are enlargeddirectly by the enlarging unit which is disposed downstream and whichcomprises the diffraction gratings VG1 and VG2. The reference symbolsused to denote the individual components are basically the same as inFIGS. 11 and 15. The reference symbols m1 and m−1 relate to the firstand—symmetrical—minus first diffraction order of a first controllablediffraction grating G1, seen in the direction of light propagation,which occur as strip-shaped illuminating regions t(x,y,RGB) for thethree colours RGB downstream of the second diffraction grating G2, asshown in FIG. 11, and which are thereafter enlarged.

The scanning and dimming solutions for illumination devices shown here,which allow efficient use of the energy emitted by the primary laserlight sources, are just examples of a much wider range of possibilities.

Light diffracting volume gratings can preferably also be used to filterthe angular spectrum of plane waves of the illumination in addition todeflecting the light, as is necessary e.g. in autostereoscopic andholographic 3D displays which require compliance with a certain angletolerance of the angular spectrum of plane waves.

The starting point here is an illuminated area having the size of thedisplay, such as the exit surface of a scanning illumination device.

The light source can for example be a fibre matrix which has outputcoupling points for secondary light sources. The fibre matrix and/or theoutput coupling points can be designed such that the exit of light iscontrollable such that at least two regions are formed which can beswitched on and off separately. The transition between the regions canalso be designed in the form of a temporally smoothened intensitytransition which serves to circumvent a flickering sensation to theobserver.

The light beams which leave the fibre matrix are collimated by a primarylens array. The lateral extent of the output coupling points of thefibre matrix are adapted to the size of the collimation lenses of thelens array such that after transmission through the lenses there is anangular spectrum of plane waves of for example 1/20° in one directionwhile it measures about 1° in the perpendicular direction. This meansthat with the same numeric aperture of the lenses in the considereddirections an individual secondary light source is 20 times as wide ashigh. The secondary light sources of the fibre matrix are thusrod-shaped.

FIG. 17 shows the entry section of such an illumination device, whichworks as a scanning illumination device. The light emitted by a primarylight source PLQ is distributed to a number of switchable paths suchthat the secondary light sources SLQ can be switched at least in groups.Segments of collimated light are emitted by the lens array L, where theangular spectrum of plane waves of these segments is determined by thesize of the secondary light sources. The desired target angular spectrumof plane waves is broadened by way of diffraction at the edges of thelenses of the collimation lens array which is disposed downstream. Ifindividual collimation lenses of the lens array have a size of 3 mm×3 mmto 5 mm×5 mm, this may possibly—in addition to an undesired diffractionbroadening of the desired angular spectrum of plane waves—also beperceived as an intensity modulation on the display which is illuminatedwith this illumination device.

This problem can for example be solved by way of angular filtering ofthe wave field which exists downstream of the lens array whichcollimates the light of the secondary light sources. This can beachieved in that for preventing the broadening of the angular spectrumof plane waves through diffraction at the edges of the lenses of theprimary collimation lens array the latter is followed in the directionof light propagation by a combination of two volume gratings for angularfiltering, as is illustrated in FIG. 17. The first volume grating VG1 israther thin (thickness d is e.g. ≤10 μm), thus exhibiting a broadangular and wavelength selectivity. Broad′ here means that the volumegrating diffracts plane waves in a larger angular range. For example, ifthe reconstruction geometry of the first volume grating VG1, which ismade of a plastic material or glass, is 0°/−45°, then an angularspectrum of the plane waves of for example ±4° is diffracted by an anglewhich corresponds with an angle of total internal reflection.

The second volume grating VG2 is rather thick, i.e. its thickness d is≥200 μm. It is made of a plastic material or glass and has areconstruction geometry of for example −45°/0°. The thickness of thegrating causes a narrow angular selectivity, which is of such naturethat only those incident light beams which lie inside the given angularspectrum of plane waves are diffracted towards the optical axis of thearrangement, while the light beams which propagate outside that angularspectrum of plane waves are transmitted without being diffracted. Themajor part of the angular spectrum of plane waves which is broadenedthrough diffraction at the edges of the lenses is thus guided out of theuseful optical path. The angular spectrum of plane waves thus has thedesired form downstream of the second volume grating VG2.

An illumination device for a direct-view display which has an angularspectrum of plane waves which is limited to ≤1/20° at least in onedirection can be created this way using a collimation lens array insteadof a single, large-area collimation lens.

The direct-view illumination device described here can for example beused in holographic 3D displays which take advantage of aone-dimensional holographic encoding method.

If a two-dimensional holographic encoding method is used, then theprocess of angular filtering according to the procedure described abovecan be performed a second time, where for angular filtering of theangular spectrum of plane waves in two perpendicular directions a secondcombination of volume gratings which is turned by 90° relative to thefirst one is disposed downstream of the first one in order to realisethe desired angular spectrum of plane waves of for example ≤1/20° in twodirections.

The illumination devices for transmissive light modulators (backlightunits BLU) described above can generally also be modified such toilluminate reflective light modulators (frontlight units FLU). Whendoing so, one problem is to keep apart the light which illuminates thereflective light modulator and the modulated light which is reflected byit.

A first option is to separate the light which illuminates the reflectivelight modulator and the modulated light which is reflected by itpolarisation-wise. For example, the illumination device which is shownin FIG. 11 can be supplemented by a retardation plate, in particular aλ/4 plate, which is disposed downstream of the two-dimensional volumegrating in the direction of light propagation (not shown). If forexample horizontal linear polarised light falls on a λ/4 plate, then itwill leave the plate having a circular polarisation. A reflective lightmodulator (not shown) which is disposed downstream in the optical pathreflects the modulated circular polarised light back towards the λ/4plate; after having passed through this plate again, it exhibitsvertical polarisation. This vertical polarised light can now passthrough the volume grating unimpeded and it can be perceived by anobserver who is situated in front of the volume grating (not shown).

Another possibility of separating the illuminating light from themodulated and reflected light is to take advantage of the angularselectivity of a light deflecting element, such as a volume diffractiongrating. A corresponding arrangement is illustrated in FIG. 18a . Itshows an illumination device in the form of a frontlight unit FLU for areflective light modulator, where a cascade of fibre-optic switchesilluminates selectable parabolic mirrors of a collimating parabolicmirror array CPMA). The light which is collimated by the parabolicmirrors is coupled into a plane light waveguide pWG through a couplingvolume grating cVG and distributed across its entire entry surface.

For example, if a transmissive volume grating of sufficient thickness isused and if the light modulator is illuminated at a sufficiently obliqueangle, i.e. for example 5° deg, then there is an ‘off-Bragg’illumination of the volume grating on the way back from the reflectivelight modulator, and this volume grating which is used to illuminate thelight modulator thus has no diffracting function. This way, the opticalpaths towards the light modulator and back from it can be kept apart.This method allows to do without the λ/4 plate shown in FIG. 18a , whichcan for example be of an apochromatic type. The latter would only benecessary if the separation of the illuminating light from the lightthat is modulated and reflected by the light modulator was achieved byusing different polarisations.

FIG. 18b shows further embodiments of line-shaped light collimationunits for injecting the light through volume gratings into planewaveguides according to FIG. 18a to scale. The reference symbols used inthe individual options have the following meanings:

-   -   FLU: frontlight unit    -   LCU: light collimation unit    -   Option B: L: lens, classic collimation (as described above)    -   Option C: PM: parabolic mirror (option with least length)    -   Option D: oaPM: off-axis parabolic mirror    -   Option E: oaPMP: off-axis parabolic mirror prism.

An input coupling volume grating which serves to inject an incidentplane wave into the core of the waveguide is always accommodated at thelower end of the plane waveguide of the illumination device forreflective light modulators. Given a sufficient thickness, the angularselectivity is sufficiently narrow for a spherical light wave whichpasses through this volume grating to be transmitted almost withoutbeing diffracted. This can be taken advantage of in order to minimisethe size of the light collimation unit. This is illustrated in the leftview in FIG. 18a , where a row of collimating parabolic mirrors isdisposed downstream of the input coupling grating and serves tocollimate the spherical waves which leave the row of fibre ends assecondary light sources. The input coupling volume grating isdimensioned such that the plane waves which are reflected by theparabolic mirrors are coupled into the plane waveguide. This is alsoillustrated in embodiment C shown in FIG. 18a . This embodiment is theshortest of all.

The other embodiments require somewhat more space and relate to classiccollimation using a lens (option B) and to a collimation using aparabolic mirror that is situated off-axis (option D) or using acombination of a parabolic mirror and a prism (option E).

The off-axis parabolic mirror prism shown in option E simultaneouslyserves as a collimator and input coupling prism, so that no volumegrating is needed for injecting the light into the plane waveguide.

This invention shall not be limited to the embodiments described hereinand can be employed in the broadest sense to realise large-area displayshaving little depth whether they use holographic or autostereoscopic ormixed methods for image generation.

Finally, it must be said that the embodiments described above shallsolely be understood to illustrate the claimed teaching, but that theclaimed teaching is not limited to these embodiments.

The invention claimed is:
 1. A holographic display comprising anillumination device, an enlarging unit and a light modulator, wherein:the illumination device comprises at least one light source and a lightcollimation unit, the light collimation unit collimating the light ofthe at least one light source and generating a light wave field of thelight that is emitted by the light source with a specifiable angularspectrum of plane waves, where the illumination device is designed as aline-shaped illumination device which generates a statistic, temporallyvariable spatial variation of the phase of the light; the enlarging unitis disposed downstream of the light collimation unit, seen in thedirection of light propagation, the enlarging unit comprising at leastone volume hologram realizing a broadening of the light wave field dueto an interaction of the light wave field with the volume hologram; andthe light modulator is disposed upstream or downstream of the enlargingunit, seen in the direction of light propagation.
 2. The holographicdisplay according to claim 1, wherein the volume hologram is designed asa transmissive volume hologram.
 3. The holographic display according toclaim 1, wherein an anamorphic broadening of the light wave field isrealized by the enlarging unit.
 4. The holographic display according toclaim 1, wherein the light wave field coming from the light collimationunit hits the volume hologram at a specifiable angle of incidence, whichis not smaller than 70°.
 5. The holographic display according to claim1, wherein a thickness of the volume hologram is chosen such that thelight wave field comprises an angular distribution of wave vectors andthat the maximum deviation of the angle distribution of wave vectors ofthe light wave field does not exceed a value of 1/20° in at least onedirection.
 6. The holographic display according to claim 1, wherein adifference in optical path length z(x, y) of the light beams of theenlarged light wave field between two defined points on the lightmodulator does not exceed a predetermined value on the encoding surfaceof the light modulator at a given coherence length of the light so thatthe difference in optical path length is small enough that said lightbeams of the enlarged light wave field are still capable of generatinginterference.
 7. The holographic display according to claim 1, whereinthe enlarging unit comprises a further volume hologram, which isdisposed downstream of the volume hologram, seen in the direction oflight propagation, and where the volume holograms of the enlarging unitare designed and disposed such that the light is deflected into twodifferent directions, where the light modulator is disposed upstream ordownstream of the further volume hologram, seen in the direction oflight propagation.
 8. The holographic display according to claim 7,wherein the light collimation unit is followed in the direction of lightpropagation by two volume holograms such that they anamorphicallybroaden the light wave field with the defined angular spectrum of planewaves in two substantially perpendicular directions.
 9. The holographicdisplay according to claim 7, wherein the volume holograms are off-axisvolume holograms.
 10. The holographic display according to claim 7,wherein one of the volume holograms is designed such that it has a fieldlens function, in addition to its function as an enlarging element. 11.The holographic display according to claim 1, wherein the light sourcecomprises a laser, laser diode, LED or OLED.
 12. The holographic displayaccording to claim 1, wherein a beam combiner is provided for combiningthe light of the at least one light source into a common optical fibre.13. The holographic display according to claim 1, wherein the lightcollimation unit comprises a primary collimation lens.
 14. Theholographic display according to claim 13, wherein the primarycollimation lens is followed in the direction of light propagation by anangular filter in the form of a volume hologram whose thickness ischosen such that the light wave field comprises an angular distributionof wave vectors and that a maximum deviation of the angle distributionof wave vectors of the light wave field does not exceed a value of 1/20°in at least one direction.
 15. The holographic display according toclaim 13, wherein the light collimation unit comprises a firstmicro-lens array which is illuminated by a collimated light wave field.16. The holographic display according to claim 15, wherein a scatteringdevice is disposed in a focal plane of the first micro-lens array, fromwhich the light propagates to a first aperture stop which is situatedimmediately downstream of it.
 17. The holographic display according toclaim 16, wherein apertures of the first aperture stop have asymmetriclateral extents in order to generate an angular spectrum of plane wavesof the light wave field with specifiable coherence properties withregard to the respective lateral extent.
 18. The holographic displayaccording to claim 17, wherein the apertures of the first aperture stopof the light collimation unit are dimensioned such that the coherenceproperties of the light wave field differ in two different directionssuch that the radiation is incoherent in the one direction, whereas itis sufficiently coherent in the other.
 19. The holographic displayaccording to claim 16, wherein a second micro-lens array is disposeddownstream of the first aperture stop in the direction of lightpropagation such that the apertures of the first aperture stop coincidewith rear focal points of the corresponding micro-lenses of the secondmicro-lens array.
 20. The holographic display according to claim 19,wherein two further aperture stops are disposed between the firstaperture stop and the second micro-lens array.
 21. The holographicdisplay according to claim 19, wherein the second micro-lens arraygenerates a segmented light wave field with an angular spectrum of planewaves with which a following light modulator which carries a holographiccode is illuminated either directly or after lateral enlargement of thelight wave field.
 22. The holographic display according to claim 15,wherein a shutter is disposed upstream of the first micro-lens array ofthe light collimation unit, seen in the direction of light propagation,where a transparency of multiple strip-shaped segments which run in thevertical or horizontal direction is controllable.
 23. The holographicdisplay according to claim 22, wherein two strip-shaped segments of anilluminating light are switched on, respectively, which run verticallyin the plane of the light modulator and which are movabletime-sequentially in the horizontal or vertical direction between theedge of the light modulator and its centre.
 24. The holographic displayaccording to claim 15, wherein selected lenses of a primary collimationlens array which is disposed upstream of the first micro-lens array ofthe light collimation unit are illuminated by a cascade of fibre-opticlight sources which are switchable by fibre-optic switches.
 25. Theholographic display according to claim 24, wherein passive light exitsare provided at the ends of optical multi-mode fibres for illuminatingselectable lenses of a primary collimation lens array which is disposedupstream of the first micro-lens array of the light collimation unit.26. The holographic display according to claim 24, wherein selectedsegments of the first micro-lens array of the light collimation unit areilluminatable by a combination of two switchable liquid crystal(LC)-based diffraction gratings, which are disposed between a primarycollimation lens that is situated downstream of the light source and thefirst micro-lens array, where an intensity of the strip-shaped segmentscould be locally variable.
 27. The holographic display according toclaim 26, wherein the combination of two switchable LC-based diffractiongratings is disposed between a primary collimation lens that is situateddownstream of the light source and the entry surface of the enlargingunit and generates two scanning strip-shaped illuminating regions whichare enlarged directly by the enlarging unit.
 28. The holographic displayaccording to claim 26, wherein the switchable diffraction gratings arepolymer dispersed liquid crystal (PDLC) volume gratings or polarisationgratings combined with switchable retardation plates.
 29. Theholographic display according to claim 26, wherein the diffractiongratings, which are disposed between the primary collimation lens of thelight source and the first micro-lens array of the light collimationunit, are designed such that off-axis optical paths are realizable aswell, in order to eliminate a 0th diffraction order of the gratings fromthe used optical path.
 30. The holographic display according to claim26, wherein the diffraction gratings, which are disposed between theprimary collimation lens of the light source and the first micro-lensarray of the light collimation unit, are designed such that anillumination of multiple surface regions of the first micro-lens arrayof the light collimation unit is realizable in a horizontal directionand in a vertical direction.
 31. The holographic display according toclaim 30, wherein for preventing the broadening of the angular spectrumof plane waves through diffraction at edges of the lenses of the primarycollimation lens array the primary collimation lens array is followed inthe direction of light propagation by a combination of two volumegratings for angular filtering.
 32. The holographic display according toclaim 30, wherein the combination of volume gratings for angularfiltering of the angular spectrum of plane waves comprises a first, thinvolume grating with a wide angular selectivity and a large diffractionangle deviating from the optical axis and a second, thick volume gratingwith a narrow angular selectivity which is designed such that the lightbeams which are incident in the region of the given angular spectrum ofplane waves are diffracted along an optical axis of the arrangement andthat the light beams which propagate outside the angular spectrum ofplane waves are transmitted without diffraction.
 33. The holographicdisplay according to claim 30, wherein a second combination of volumegratings, which is turned by 90° relative to a first combination ofvolume gratings, is disposed downstream of the first combination ofvolume gratings to be able to realize an angular filtering of theangular spectrum of plane waves in two perpendicular directions.
 34. Theholographic display according to claim 24, wherein selected segments ofthe first micro-lens array of the light collimation unit areilluminatable by a combination of two diffraction gratings, where thefirst diffraction grating is of a switchable type and where the seconddiffraction grating is disposed upstream of the first micro-lens arrayand designed in the form of an angle-selective volume grating, wherethis angle-selective volume grating serves to realize requireddeflection angles for at least one light wavelength with the help of afixed inscribed diffractive structure.
 35. The holographic displayaccording to claim 24, wherein selected segments of the first micro-lensarray of the light collimation unit are illuminatable by a combinationof two diffraction gratings, where the first diffraction grating is of aswitchable type and where the second diffraction grating is disposedupstream of the first micro-lens array and designed in the form of avolume grating, where the volume grating comprises multiple strip-shapedsegments which are designed such that the light which hits thestrip-shaped segments at an angle that increases as the distance to theoptical axis of the arrangement becomes larger is diffracted into adirection that is parallel to the optical axis.
 36. The holographicdisplay according to claim 24, wherein the lenses of the primarycollimation lens array, which is disposed upstream of the firstmicro-lens array of the light collimation unit, are illuminated bysegmented plane waves whose angular spectrum comprises an angulardeviation of about 1/20° in one direction and of about 1° in aperpendicular direction.
 37. The holographic display according to claim24, wherein the light modulator is of a reflective or transflective typeand/or where the illumination device is designed and supplemented byoptical components which modify a polarization of the light such thatthe illumination device illuminates the active area of the reflective ortransflective light modulator with light of a specifiable polarization.38. The holographic display according to claim 37, wherein the lightmodulator is of a reflective or transflective type and/or where theillumination device is designed and supplemented by line-shaped lightcollimation units which collimate the light which is emitted by a linearrangement of switchable secondary light sources and couple it into aplane waveguide either directly or through angle-selective deflectionelements.
 39. The holographic display according to claim 24, wherein thelight modulator is of a reflective or transflective type and/or wherethe illumination device is designed and supplemented by plane waveguidescombined with angle-selective deflection elements such that the lightdiffracting function of the deflection element will only be effectivewhen light is coupled into the waveguide and when the light is coupledout to illuminate the light modulator, but not when light which ismodulated and reflected by the light modulator is on its way back. 40.The holographic display according to claim 1, wherein the lightmodulator is of a transmissive, reflective or transflective type and/orwhere the illumination device is designed and dimensioned such that itilluminates an active area of the light modulator substantiallyhomogeneously.
 41. The holographic display according to claim 1, whereinat least one parameter of the light collimation unit is modifiable inorder to generate a specifiable angular spectrum of plane waves of thelight wave field downstream of the at least one volume hologram.
 42. Theholographic display according to claim 1, wherein the at least onevolume hologram is designed such that it suppresses disturbing portionsof radiation or diffraction orders for an observer who looks at thedisplay.
 43. The holographic display according to claim 1, wherein thelight collimation unit has a line structure, where each line has atleast two light sources which can be switched independently of eachother and, at the exit, at least two collimating refractive lenses, andwhere these lines are arranged side by side along an edge of asubsequently arranged two-dimensional enlarging unit such that theyilluminate the entire surface of the latter.
 44. The holographic displayaccording to claim 43, wherein a wedge-shaped light waveguide device isprovided for illuminating a two-dimensional enlarging unit whichcomprises a volume grating, wherein the volume grating is attached tothe side of the wedge-shaped light waveguide device which is situatedsubstantially perpendicular to its light entry surface and serves toenlarge a segmented plane wave field which is emitted by the lightcollimation unit and to direct it at the surface of a light modulatorwhich is disposed further downstream.
 45. The holographic displayaccording to claim 43, wherein depending on the number of lines whichare arranged side by side in the light collimation unit and the numberof light sources which can be switched independently of each other ineach line, the thus formed illumination device has a matrix ofindependently switchable illumination segments.